The invention relates generally to a method for transmission of digital signals, and more particularly to a method for the transmission of digital signals in the AM bands with noise suppression.
In digital broadcasting transmission, it is necessary, for reasons of efficiency, to use high-level modulation methods such as 64 APSK (Amplitude Phase Shift Keying). However, such high-level modulation methods have the disadvantage that even small noise signals cause the corruption of the received data. This problem is solved, on the one hand, in that the transmission power is set suitably high, so that the influence of the smaller noise signal is reduced. On the other hand, noise suppression systems are used at the receiving end; such noise-suppression systems filter out the noise signal from the received signal. This has the advantage that the transmitter can be operated at a lower power.
Described in the article xe2x80x9cAdaptive Noise Cancelling: Principles and Applicationsxe2x80x9d, B. Widrow et al., Proceedings of the IEEE, VOL. 63, No. 12, December 1975, pages 1692 to 1716 is a structure with two signal inputs at the receiving end. Present at the first signal input is the desired signal including a noise. Present at the second signal input is the noise alone. This noise present at the second input is adaptively filtered and subtracted from the received signal at the first input. The result is then the desired signal requiring further processing.
This noise suppression system has the disadvantage that use must be made of a complex, and therefore expensive, adaptive filter. Furthermore, it is necessary for the noise signal alone to be constantly measurable. This system cannot be used for broadcasting applications.
A object of the present invention, therefore, is to provide a method for the transmission of digital signals in which the suppression of noise signals is possible using simple means.
The present invention therefore provides a method for transmitting digital signals comprising: modulating the digital signals onto a carrier; transmitting a test sequence and the digital signals in a periodic alternating fashion, the test sequence having essentially no direct component; averaging the test sequence at a receiver end so as to detect a noise carrier; and subtracting the noise carrier from the digital signals at the receiver end.
The fact that, at the receiving end, time-periodic measurements of a noise are carried out and that noise signals determined therefrom are subtracted from received signals allows a simple construction which, in particular, does not require an adaptive filter. In addition, it is not necessary for the noise signal to be constantly present in a form separate from the transmitted data stream.
It has proved particularly advantageous to precede each transmitted data block with a sequence of binary data, referred to as a test sequence, said binary data being modulated at a low level, for example by means of 2PSK modulation. Preferably, the test sequence is in the form of a pseudo-random sequence which is sufficiently long and has no direct component. By means of the aforementioned low-level modulation process, it is possible in simple manner at the receiving end to determine, first, the frequency and, second, the phase thereof, this being required for the processing of the data blocks.
Preferably, in order to determine a noise signal, an interval of the test sequence is selected and averaged. The fact that the actual binary data cancel each other out during averaging means that merely the noise signal is left, which is then subtracted from the received data signal.
In a further advantageous embodiment of the present invention, the transmitted signal sequence has so-called gaps in which the transmitter transmits nothing. These gaps are repeated periodically and are used by the receiver to measure noise signals.